88EM8040/88EM8041
Datasheet
5.3
5.3.1
Current Sensing and Over Current Protection
Current Sensing Through ISNS Pin
The voltage drop on the current sense resistor should be kept very small in order to reduce the
power consumption on the sense resistor. In flyback topology, the drain to source current flows
through the transformer, MOSFET and current sense resistor (R
sen
). This is shown in
The
average current mode control single stage solution requires two signals of flyback: the peak current
signal to avoid the transformer saturation including a short circuit condition, and the average current
sense signal to achieve the right PFC operation. The voltage drop (V
sen
) across resistor (R
sen
)
represents the flyback peak current signal. The voltage of (V
CS
), after R
CS
and C
CS
low pass filter,
represents the average current signal of the primary side of the flyback converter.
Figure 23: Current Sensing Circuit
V
DCin
C
sn
C
in
V
sen
D
sn
Q1
R
sen
R
cs
C
cs
OCP SW S GND
I SNS
V
cs
Drain
N
p
R
sn
D
R2
V
OUT
N
S2
88EM8040
VIN
/8041
VDD
FB
P GND
The resistor (R
sen
) should be designed such as the example in
where R
sen
is designed for a
90W adaptor. The specification are: output power = 90W, input voltage range = 85-264V, output
voltage = 20V, output current= 4.5A, 30% margin of over current on top of the normal current.
Doc. No. MV-S104983-01 Rev. A
Page 34
Document Classification: Proprietary
Copyright © 2009 Marvell
October 5, 2009, Preliminary
MARVELL [ MARVELL TECHNOLOGY GROUP LTD. ]
